It is well known that reducing the hydrophobicity of a membrane is advantageous, as it reduces the fouling tendencies of the membrane. This naturally leads to a problem, as the least expensive and most stable membrane forming materials are polymers that are quite hydrophobic. There is also an advantage in making a membrane that is hydrophilic and therefore easily wettable with water, as this makes use of the membrane simpler and obviates the need for wetting solvents.
To decrease the hydrophobicity inherent to most polymeric membrane materials, it is known either to chemically modify the surface and pore-walls of a support member or, alternatively, to coat the walls of the pores in the support with a hydrophilic layer, the layer usually being polymeric in nature. The coated hydrophilic layer improves the affinity of the composite material towards water, increasing its wettability and, in some cases, making the membrane completely wettable by water.
Early efforts in the art to adhere the hydrophilic layer to the support included activating the walls of the pores in the support (for example with a plasma treatment) such that the coating is chemically attached to the pore-walls [Nystrom M. et al., Journal of Composite material Science. 60(1991)275-296]. These coatings could also be made by polymerizing a mixture of monomers within the support member to be coated under conditions that the thus formed polymer is grafted to the walls of the substrate. Under certain conditions where there is no crosslinking or low degrees of crosslinking of hydrophilic and particularly charged grafted polymers, the grafted layer can become hydrated and expand in thickness to essentially fill the pores of the substrate. Such composite materials were found to be very hydrophilic and readily wet with water.
A further advance in the art was made when it was discovered that formation of a crosslinked polymer can be used as a surface coating which has a superior combination of properties, including heat stable biomolecule resistance adsorptive properties, resistance to strong alkaline solutions, and low levels of extractable matter [Charkoudian J. and Soice N. P. WO 02/087734 A1].